Precision Rogowski coil and method for manufacturing same

ABSTRACT

An improved Rogowski coil is formed on a toroidal core made of a thermoplastic or other moldable material, the core having a preferably continuous groove or grooves extending around the core. The grooves correspond in size to magnet wire which registers within the grooves, thus controlling the specific location of the wires. The grooving may be helical. A return loop can be provided for return path cancellation, or a reverse winding can be added in a direction opposite to the direction of advancement of the main coil. In using the return loop, a resistive network can be added to improve the cancellation of the return path due to the effect of geometries. In addition, it can compensate for thermal and other variations.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional application of U.S. patent applicationSer. No. 11/051,232 filed on Feb. 4, 2005 now U.S. Pat. No. 7,227,441,and entitled “Precision Rogowski Coil and Method for ManufacturingSame”, naming Veselin Skendzic and James R. Kesler as inventors.

BACKGROUND OF THE INVENTION

The present invention concerns Rogowski coils. More particularly, thepresent invention concerns the structure and manner of making a Rogowskicoil.

Rogowski coils are well known electrical devices finding use today formeasurement of magnetic fields and electrical currents. They have beenresearched over the past century and are well known to the scientificliterature. Their origin traces to the invention circa 1912 of theRogowski coil by W. Rogowski and W. Steinhousen. The device is usefulfor measuring electrical currents and operates on the basis of amagnetic field integration performed across a closed contour being equalto the current flowing through the contour. The coil provides a voltageoutput proportional to the time derivative of the current (di/dt) ratherthan a current output like other current transformers.

Rogowski coils are popular because of their dynamic range and linearity.However, though theoretical requirements are known, manufacturers stillneed ways to provide a high quality coil that is both economical tomanufacture and which is satisfactory for precise current measurements.The device (coil) should be insensitive to external influences,insensitive to the measured primary conductor position, and retain highprecision (in the order of 0.3% or better) over its lifetime and acrossa wide temperature range (nominally −40 to 70 degrees Centigrade).

One known approach to making a Rogowski coil involves using a printedcircuit. U.S. Pat. No. 5,414,400 entitled “Rogowski Coil” discloses aRogowski coil made on a printed circuit plate provided with a circularcut-out. The coil is implemented by metal deposits on each of the twofaces of the plate extending along radii, with electrical connectionsbetween the radii on one face and those on the opposite face beingachieved via plated-through holes passing through the thickness of theplate.

U.S. Pat. No. 5,442,280 discloses a method for manufacturing a printedcircuit board-based Rogowski coil. The disclosed geometry provides veryhigh turn density resulting in very high sensitivity. While highsensitivity is very desirable when measuring low frequency currents(50/60 Hz power system related), the patent fails to provide adequatemeans for external field cancellation. This problem is reported in U.S.Pat. No. 6,624,624 and is caused by inadequate handling of the coilreturn path.

A similar problem applies to the design reported in U.S. Pat. No.6,313,623 (by the current inventor) in which two closely spaced coilswith counter rotation are used to perform partial return pathcompensation.

Further attempts to design precision Rogowski coils are disclosed inU.S. Pat. No. 6,624,624. Attempts to provide improved return pathcancellation resulted in significantly reduced coil densities, makingthe design less appropriate for low frequency current measurementapplications. In addition, although significantly improved, all reportedgeometries suffer from Z-axis (board thickness) related sensitivitycontour effects with an error cancellation (return) path normally offsetin the direction of the Z-axis (board thickness).

J. D. Ramboz in “Machinable Rogowski Coil, Design and Calibration,” IEEETransactions on Instrumentation and Measurement, Vol. 45, No. 2, (April1996) pp 511-15 reviews traditional designs for Rogowski coils anddiscusses a “machinable” Rogowski coil constructed using machinableceramic material to make a toroidal coil with a rectangular crosssection. A thin, electrically conductive coating is then applied to thecoil, totally encapsulating the ceramic core. Next, thin lines of theconductive material are removed by laser machining methods in a patternwhich leaves coils as bands of conductive material located radiallyaround the core. Each turn or band was connected to the next turn by asuitable indexing.

U.S. Pat. No. 6,300,857 for “Insulating Toroid Cores and Windings”discloses a configuration to improve the winding of precise conventionaltransformer coils and includes an insulating jacket around a magneticcore. The insulating jacket includes plural protrusions around the core,the protrusions demarking various segments of the toroid. For example,the toroid may be divided into six evenly spaced sections, eachoccupying approximately 60°. At the edges of each section, there is aprotrusion. The protrusions maintain the placement and spacing ofwindings within each section.

An object of the present invention is to provide a precision Rogowskicoil with its winding geometry defined and controlled at the time ofmanufacture, with improved dimensional stability maintained throughoutits lifetime.

SUMMARY OF THE INVENTION

A precision Rogowski coil according to certain aspects of the presentinvention includes a generally toroidal (or toroid-like) core havinggrooving thereon in which to register the wire (or other conductivematerial) turns of a main coil. Illustratively the core is made of amoldable material having grooves therein in a pattern corresponding tothe main coil winding. Preferably the grooves are helical, and the coilwire or conductor fits into the grooves on the core. A conductor otherthan wire can be used, such as a deposited conductor. A return path orreverse winding are also provided, and various structures and methodsaddress return path cancellation and thermal and other variations.

Invented manufacturing techniques include molding a core having groovingon its exterior and then winding wire in the grooving or otherwiseforming a conductive winding using the grooving to control the locationof the winding on the core.

BRIEF DESCRIPTION OF THE DRAWINGS

In describing preferred embodiments of the present invention, referenceis made to accompanying drawings wherein like parts bear similarreference numerals and wherein:

FIG. 1A is a diagram of a top view of a toroidal coil core, prior towinding wire thereon, with some of the grooving shown, and embodyingvarious aspects of the present invention;

FIG. 1B is a diagrammatic side view of the core of FIG. 1A, and showinga circumferential wire loop wound thereon for return path cancellation;

FIGS. 2A and 2B are sketches of two alternate constructions of the corewith representative constant pitch helical winding wound thereon using acircumferential loop for return path cancellation;

FIG. 3 is an electrical circuit diagram of a resistive networkapplicable to Rogowski coils of the present invention for effectivereturn path diameter (radius) compensation;

FIGS. 4A and 4B represent another embodiment of a toroidal core withmodified grooving, forming a Rogowski coil having a winding that isradial on three sides of the toroid with coil advancement restricted tothe forth (outer) side;

FIG. 5 represents an alternate construction of a toroidal core embodyingaspects of the present invention;

FIG. 6 is used in describing an alternate coil manufacturing method inwhich the grooved toroidal core is used as a substrate for conductivematerial being deposited within the grooves;

FIG. 7 is an electrical circuit diagram of a resistive networkapplicable to Rogowski coils of the present invention for temperatureand gain compensation; and

FIG. 8 represents another embodiment of the current invention in whichthe grooves are used to support a second coil wound on top of the firstcoil, but with an opposite direction of coil advancement.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1A illustrates a core 10 shaped generally as a toroid. The term“toroid” often connotes a “doughnut” shape but the present inventiondoes not require a “doughnut” shape for the core 10. It will beunderstood from the illustrative drawings and the present descriptionthat a cross section cut through a diameter or radius of core 10 can beeither circular, substantially rectangular, or otherwise, e.g. oval,oblong, or some other shape. That is, core 10 may have the geometricshape of a solid formed by rotating (orbiting) a closed form, be it acircle, square, rectangle, oval, oblong, or irregular shaped closedline, around 360 degrees of a circle. Thus, FIG. 2A shows a Rogowskicoil according to certain aspects of the present invention where thecore has generally “flattened” faces (like an annulus), which (but forthe grooving) could be formed geometrically by rotating a rectangle withrounded corners around 360 degrees of a circle. FIG. 2B on the otherhand shows another core according to aspects of the present invention,where the faces are not flattened but instead are rounded with acircular cross section (somewhat like an O-ring). That geometric form(again, without the grooving) could be obtained by rotating a circlearound 360 degrees of a larger circle. Hence, the term “toroid” as usedherein is used in a broad context and is intended to embrace all suchgeometries.

The wires for a precision Rogowski coil will be wound on core 10. Core10 may vary widely in dimensions, illustratively on the order of 0.25inch in outside diameter (or less) to eighteen or twenty inches, ormore. Preferably core 10 comprises a non ferro-magnetic, electricalinsulator, preferably one with a minimal coefficient of thermalexpansion, and more preferably a moldable polymeric material.Illustratively, core 10 may be made of Thermoset Polyester with thermalexpansion coefficient in the order of 9 ppm/° C., glass reinforcedthermoplastic polyester PET (36 ppm/° C.), glass reinforced epoxy (36ppm/° C.) or ABS/polycarbonate (67 ppm/° C.). It is understood thatother materials can be used to achieve a specific cost/performancetarget without diverging from the spirit of this invention.

Core 10 is patterned with precision grooves 12 around its exterior.Although FIG. 1A illustrates the patterning in a small continuoussection, it is to be understood that the patterning extends, in thepreferred embodiment, around the full 360° (2π radians) as shown inFIGS. 2A, 2B, 4A, 4B, 5, 6 and 8. In the first embodiment, groove 12 isa single, continuous helical groove as shown in FIG. 2B. The groovepattern may be molded with uniform, predetermined characteristics sothat each core 10 made from the mold has precisely the same shape andthe same precision grooving. Alternatively, groove 12 could be placed onthe core by a machine process. It is convenient to refer to this patternas “grooving,” a “groove,” or “grooves” since the singular or plural canbe used to describe this feature which may comprise, as indicated, asubstantially continuous groove that wraps about the toroid in aclockwise or counterclockwise direction. Hence, when viewed from a planview, as in FIG. 1A, one sees what appears to comprise a plurality ofgrooves which indeed (in one embodiment) comprise a single groove havingmultiple turns, thereby appearing to comprise plural grooves.

Preferably grooving 12 has a uniform pitch around the toroid. Preferablythe dimensions of grooving (i.e., its width and depth) correspond insize to magnet wire to be used for wrapping around the core. The magnetwire used for the coil and return path conductor can be wound on thetoroidal core in well-known fashion using well-known equipment for thistask. The coil may comprise a large number of turns, illustratively 1000turns, although no upper or lower limitation in the number of turns isimplied. During the winding process, the wire of the main coil seats inthe grooves 12. That is, with respect to FIG. 1A, wire may be woundaround the toroid so that it “enters the page” on the outside of thecore 10 and “comes out of the page” in the central opening of thetoroid, or vice versa. By using this grooving, the coil geometry isdefined and controlled at the outset, and the resulting Rogowski coilwill be dimensionally stable throughout its lifetime. Following windingof magnet wire, a varnish based coating can be added to furtherstabilize the coil.

Return path cancellation can be achieved in various ways. One way is toprovide a return loop. Illustratively, a circumferential groove 14 maybe machined, molded, or otherwise provided around the outside edge ofcore 10, as seen in FIG. 1B. A return loop 16 can be positioned in suchgroove 14. The return path (return loop 16) is to null out, to theextent possible, the effect of the coil wound about the toroid. Thus, ifthe coil advances around the toroid in a clockwise direction, thenreturn loop 16 must be wound in the counterclockwise direction, or viceversa. Typically, the return loop is wound around the circumferencefirst, and then the coil is wound over toroid on which the return pathconductor is already in place. This is a first technique for return pathcancellation.

Turning now to FIGS. 2A, 2B, and 3, to make the Rogowski coil moreprecise, the cancellation of the return path conductor may be improvedby the addition of a resistive network. FIG. 2A is a view of a Rogowskicoil, which comprises main wire turns 18 and the return loop 16.Electrically, the coil has points 1, 2, and 3. Point 1 is the end of thereturn loop 16. Point 2 is the beginning of the main coil 18. Point 3represents the point at which the main coil 18 connects to the returnloop 16.

The main coil 18 will have an effective advancement path radiusrepresented by r1 in FIG. 1A. As visible from FIG. 1A, for a uniformlywound helical coil, effective advancement path r₁ will be positionedwithin the coil, half way between the inner radius r_(i) and the outerradius r_(o) of the coil. A similar situation occurs depth-wise, thusmaking the effective advancement path effectively positioned in thecenter of the core 10 used to support the winding. Return loop 16 islocated generally at the outer edge of the core 10, except it sitswithin groove 14 so that its actual radius is slightly less than themaximum radius of the toroid. In FIG. 1A, r2 denotes the actual radiusof the return loop 16. The return loop radius r2 is generally largerthan effective radius r1 with the result that return path cancellationis suboptimal. Other parameters, namely the z-axis position andconcentricity of the two paths (“front” and “back” of the toroid) arewell matched, making it possible to use a passive circuit basedtechnique for return path effective diameter (radius) adjustment.

An effective diameter adjustment technique is shown in FIG. 3. The basicmathematical relationship between individual coil outputs for a magneticfield parallel to the axis of the toroid is given in Equation 1 below,where the notation V_(Desired) is used to indicate the coil outputresulting from the main coil advancement path governed by the effectiveradius r₁. Since radius r₂ is different than r₁, the return pathcompensation coil output V_(Actual) will in general be different, andthe ratio between the voltages will be proportional to the ratio of thesquares of the two radii. By advantageously positioning the return coilon the outside of the effective advancement path radius r₁ of the maincoil (r₂>r₁), it is possible to ensure that V_(Actual)>V_(Desired)making it possible to use a simple resistive divider (highly reliablepassive network) to match exactly the desired compensation path outputto the actual output of the coil advancement path. The resistive divideris used to effectively reduce radius r₂ with the objective of matchingit to the radius r₁.

$\begin{matrix}{\frac{V_{Desired}}{V_{Actual}} = \frac{r_{1}^{2}}{r_{2}^{2}}} & (1) \\{V_{Desired} = \frac{V_{Actual} \times R_{2}}{R_{1} + R_{2}}} & (2) \\{R_{1} = {\frac{\left( {r_{2}^{2} - r_{1}^{2}} \right)}{r_{1}^{2}} \times R_{2}}} & (3)\end{matrix}$

Equation (2) above describes the matched state relationship. Simplealgebraic manipulations allow equations (1) and (2) to be used todetermine the ratiometric relation between gain matching resistors R1and R2. The final relation is given in equation (3). It will beunderstood that by changing the coil geometry (thus affecting theeffective radii r₁ and r₂ by making r₁>r₂), it may become necessary toattenuate the output of the main coil instead of the return path output.Such modification is anticipated and does not deviate from the spirit ofthis invention.

The exact value of matching resistors is less significant than theirratio, with typical values ranging between 1 and 1000 ohms. Ifadvantageous, the resistor value can be selected to match the attachedtransmission line impedance. The resistor value should be kept below1000 ohms to reduce the resistor-generated noise contribution. Theresistive network can be placed in close proximity of the coil, bysoldering resistors R1 and R2 directly to the return (compensation) coiloutput. If required, a separate set of conductors can be used to bringboth the output of the main coil and the return coil to a remotelocation (up to 100 m) equipped with resistors R1 and R2.

Referring to FIG. 4A, and 4B, a modification of this first approach forcancellation is to provide essentially radial grooves 12 at firstlocations 21 (upper face), 22 (inside face), and 23 (lower face) of thetoroid, and to provide sequential non-radial grooving 12 a (see FIG. 4B)at second locations 24 radially outward of the radial grooving, therebyto concentrate the coil advancement at the second locations 24. It willbe appreciated that in FIGS. 4A and 4B, the second locations 24 arelocated toward or on the outside edge of the toroid core 10. As will beappreciated, the grooves 12 a are not radial, but are pitched 32, andremain symmetrical. They include a radial component located closer tothe center of the toroid and a non-radial component 32 located outward,close to the outer edge of the toroid as shown in FIG. 4B. Byconcentrating all of the coil advancement on the outside edge theadvancement path, radius r1 is increased and can be brought very closeto the return loop radius r2, thus alleviating (and in less demandingapplications, totally eliminating) the need for resistor basedcompensation.

A third technique for return path cancellation is to provide, instead ofthe single loop 16 in groove 14, a second layer of winding, that is, areturn winding, with a constant pitch adjusted to avoid physicalinterference between the two layers. The pitch of the second windinglayer need not match the winding pitch of the first layer and canconveniently be reduced to as little as ten turns, illustratively.Further reduction lower than ten turns is counterproductive due to thepotential for increased sensitivity to the measured conductor position.Thus, after winding a first coil using the grooving 12 as a guide toregister the wire turns, a return winding is made on top or next to thefirst one, but in the opposite direction of coil advancement. Oneembodiment of this invention is shown in FIG. 8. The windings areassumed to cover the entire circumference of the toroid, as indicated bythe arrows in the figure.

Through use of the patterned thermoplastic, a high quality coil can bemanufactured economically and still be essentially insensitive toexternal influences, insensitive to measured primary conductor position,and will retain high precision on the order of 0.3% over its lifetimeacross a wide operating temperature range, illustratively from −40° C.to 70° C. It will be understood, furthermore, that the core need not bethermoplastic material but can be an insulator manufactured according toany known technology such as molding, machining, and other forms ofmanufacturing well known in that art.

Nevertheless, if there are effects of temperature, temperature and gaincompensation techniques are applicable to the precision wound toroidaccording to the present invention. Return path cancellation as setforth above is used to compensate for the effect of coil advancement. Anactual Rogowski coil implementation will exhibit additional sources oferror. The most pronounced additional errors are: (1) temperaturedependence of the coil output; (2) individual coil gain differencescaused by the manufacturing process variations; and (3) capacitivecoupling based errors from neighboring high voltage conductors.

Temperature dependence is present if the material used to form the corefor the Rogowski coil changes physical dimensions (expands andcontracts) with temperature variations. It is very common for materialsto expand with increasing temperature, causing in turn an increase ofthe Rogowski cross sectional area. Since the Rogowski coil output is adirect function of the cross sectional area, as given in Equation (4)below, it will increase as the material used to support the coilexpands.

$\begin{matrix}{V_{(t)} = {\mu_{0}S\frac{\mathbb{d}i}{\mathbb{d}t}}} & (4)\end{matrix}$

-   -   where: μ₀=permeability of free space (4π*10⁻⁷ Vs/Am)    -   S=total cross sectional area of the coil    -   i=measured primary current    -   t=time

Careful selection of core material is used to minimize the thermalexpansion coefficient, thus significantly reducing the temperaturedependency of the coil gain. Preferred core materials have beenmentioned above. While dimensionally stable materials (with acoefficient of thermal expansion (“CTE”)<5 ppm/° C.) are available,material cost increases with tighter CTE specifications. If the coil isto be formed on a material with a non-zero or negligible CTE, it will bedesirable to provide localized compensation for temperature effects.

Referring to FIG. 7, such compensation can be achieved by adding athermally sensitive compensation resistor (R3) and combining it with atemperature stable resistor (R4). When combined, resistors R3 and R4form a temperature compensating divider, which is added to the main coiloutput. By carefully adjusting individual value of the resistors R3and/or R4 it is also possible to compensate for the individual gainvariation among multiple coils. Gain compensation is normally performedduring coil manufacturing (factory calibration) by trimming one or bothlegs of the output divider. Individual coil gain is determined bypassing known electrical current through the coil opening, and recordingthe voltage present at the coil output, or by comparing the coil outputwith a known calibration artifact.

It will be appreciated that grooves 12 need not extend in a singlehelical pattern around core 10. In the event that, for example, onedesired to segment the coil, the grooves 12 could be segmented asdesired, illustratively providing left and right segments each occupyingsubstantially 180° of arc, or three segments each having substantially120° of arc, or four segments having 90° of arc each, and so forth.

It is preferred that the grooves 12 extend substantially continuouslyaround the toroidal core 10 in helical fashion. One discontinuityalready occurs where groove 12 intercepts the circumferential groove 14.However, further discontinuities could be allowed, with groove beingessentially reduced to simple notches positioned at the four edges ofthe toroid. Thus, while grooving may extend around 360° of arc of thecoil (in the plane represented in FIG. 1), a variation may truncate thegrooving.

FIG. 5 shows one such variation wherein grooves 12 are discontinuous butnevertheless aligned in the helical or other patterns described herein.Grooves 12 may, illustratively, comprise first portions 40 located onthe face of the toroid, second portions located around an outsideperimeter (not shown in FIG. 5), third portions located around an insideperimeter (not shown in FIG. 5), and fourth portions 42 located on theopposite face of the toroidal coil 10.

In yet another aspect of the invention, the main coil can bemanufactured by using conductive material deposited into the grooves ofthe toroid. This approach requires three steps: manufacturing of thegrooved toroid, application of the conductive layer covering the entiresurface of the toroid, and removal of the conductive material from theelevated ridges, thus forming the coil by leaving only the conductivematerial within the grooves. This approach is illustrated in FIG. 6wherein the entire surface of the grooved toroid 10 is made conductiveusing electrochemical deposition (electroplating), vacuum deposition, orby extruding conductive polymer into the grooves 12. The coil is formedby subsequently machining (sanding, grinding) the outside surfaces ofthe toroid, thus forming the insulated ridges 60 in between theindividual loops of the main coil 12. The return coil is realized inaccordance with the previous descriptions and is not shown in FIG. 6.

While above disclosures describe Rogowski coil geometry realized on agrooved toroidal core, it is to be understood that basic toroid geometrymay be substituted with an ellipse, rounded corner parallelogram orother shape necessary to meet specific application requirements.

It will be evident that other modifications or variations can be madewithin the scope and spirit of the present invention. While theinvention has been described in conjunction with specific illustrativeembodiments, those embodiments are not intended to limit the scope ofthe present invention, the scope of which is set forth in theaccompanying claims which are intended to embrace alternatives,modifications, and variations which fall within the spirit and scope ofthe invention.

1. A coil comprising: a generally toroidally-shaped core; grooving onthe exterior surface of said core; and a conductive coil extending aboutsaid core, said conductive coil being registered in said grooving thecoil constituting a main coil; a return loop wrapped generallycircumferentially about the core; and, a resistive network electricallycoupled to the return loop and the main coil.
 2. The coil of claim 1,said return loop wrapped on top of said main coil in a reverse directionof advancement as said main coil.
 3. A coil comprising: a generallytoroidally-shaped core comprising a non-magnetic insulating material;grooving on the exterior surface of said core, at least a portion ofsaid grooving being helical, said grooving being configured to receivewire; and a wire coil wound in a helical configuration about said coreand registered in said grooving advancing in a first direction, the coilconstituting a main coil; a return loop wrapped generallycircumferentially about said core and advancing in a second direction;and, a resistive network electrically coupled to the return loop and themain coil.
 4. The coil of claim 3, said return loop wrapped on top ofsaid main coil in a reverse direction of advancement as said main coil.5. A coil according to claim 1 wherein said resistive networkcompensates for differing radii as between a return path and said coileffective advancement path radius.
 6. The coil according to claim 1wherein said resistive network compensates for thermal effects on thecoil.